Retirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Amortization
Prior service cost (credit)	$ 38.4
Net actuarial loss	60.1
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 70.9	$ 90.1	$ 91.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Interest cost	$ 135.6	$ 125.6	$ 136.4
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Expected Return (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Expected return on plan assets	$ (230.7)	$ (241.3)	$ (244.8)
Amortization
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Amortization of Prior Service Cost (Credit), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Prior service cost (credit)	$ 0.6	$ 1.4	$ 0.9
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Immediate Recognition of Actuarial Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Net actuarial loss	$ 59.4	$ 141.4	$ 147.3
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Settlement and Curtailment Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Settlement and curtailment charges	$ 38.6	$ 39.8	$ 0.0
Net periodic benefit cost (income)	74.4	157.0	130.9
Other Postretirement Benefits
Components of net periodic benefit cost
Service cost	$ 0.8	$ 1.2	$ 1.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Interest cost	$ 1.3	$ 1.2	$ 1.6
Expected return on plan assets	$ 0.0	$ 0.0	$ 0.0
Amortization
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Amortization of Prior Service Cost (Credit), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Prior service cost (credit)	$ (0.8)	$ (5.4)	$ (5.4)
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Immediate Recognition of Actuarial Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Net actuarial loss	$ 0.7	$ 1.1	$ 1.4
Settlement and curtailment charges	0.0	0.0	0.0
Net periodic benefit cost (income)	$ 2.0	$ (1.9)	$ (1.4)